TWI847971B - Synthetic lined crucible assembly for czochralski crystal growth - Google Patents

Abstract

A method of manufacturing a crucible assembly having a shell and a liner is disclosed. The method includes forming the shell using a casting process. The shell includes silica and has an inner surface and an outer surface. The method also includes forming the liner on the inner surface of the shell. The liner is formed of synthetic silica.

Description

Synthetic lined crucible assembly for Czochralski crystal growth

The present invention relates generally to systems and methods for producing ingots of solar grade or semiconductor materials, and more particularly to crucible assemblies including synthetic linings for use in such systems and methods.

Crystalline silicon solar cells currently contribute a large portion of the total supply of photovoltaic (PV) modules. In the standard Czochralski (CZ) process, polycrystalline silicon is first melted in a crucible such as a quartz crucible to form a silicon melt. A seed crystal of a predetermined orientation is then lowered into contact with the melt and slowly withdrawn. By controlling the temperature, the silicon melt at the seed-melt interface solidifies onto the seed crystal having the same orientation as the seed crystal. The seed crystal is then slowly raised from the melt to form a growing crystal ingot. In the known CZ process, known as batch CZ (BCZ), the entire amount of charge material required to grow a silicon ingot is melted at the beginning of the process, crystals are pulled from a single crucible charge to substantially deplete the crucible, and the quartz crucible is then discarded. Another method of economically replenishing a quartz crucible for multiple pulls in one furnace cycle is continuous CZ (CCZ). In CCZ, solid or liquid feedstock is continuously or periodically added to the melt as the crystal grows and thereby maintaining the melt at a constant volume. In addition to spreading the crucible cost over several ingots, the CCZ process also provides excellent crystal uniformity along the growth direction.

Cast silica crucibles formed from lower grade natural silica are generally not used in Czochralski-based processes (CZ, BCZ and CCZ processes) because lower grade natural silica has a high total impurity content. In contrast, cast crucibles or polysilica cast crucibles are typically used to make polycrystalline silica photovoltaic cells due to the higher total impurity content. The higher total impurity content of lower grade natural silica comes from impurities naturally present in the silica and from impurities added to the silica during the casting process to bond the silica into the cast crucible form. A higher total impurity content of the cast crucible increases the impurity contribution from the crucible into the melt and into the final product.

In contrast, crucibles used in Czochralski-based processes (such as arc-fused crucibles) are formed from higher grade, more expensive natural silicon dioxide, which have lower overall impurity content than cast crucibles formed from lower grade natural silicon dioxide. Ingots formed from such crucibles have lower impurity content than ingots formed from cast crucibles. Therefore, there is a need for lower cost crucibles for use in Czochralski-based processes that reduce the impurity contribution from the crucible.

In addition, known crucibles used in Czakowski-based methods suffer from limited design flexibility and limited crucible life. Therefore, there is a need for a crucible for use in Czakowski-based methods having improved design flexibility and improved crucible life, for example, to extend the length of a furnace cycle.

This prior art section is intended to introduce the reader to various aspects that may be related to the various aspects of the present invention, which are described and/or claimed below. It is believed that this discussion helps to provide the reader with background information to promote a better understanding of the various aspects of the present invention. Therefore, it should be understood that these statements should be read in this light and not as an admission of prior art.

The first aspect is a method of manufacturing a crucible assembly having a shell and a liner. The method includes forming the shell using a casting method. The shell includes silicon dioxide and has an inner surface and an outer surface. The method also includes forming the liner on the inner surface of the shell. The liner is formed of synthetic silicon dioxide.

Another aspect is a crucible assembly for growing a crystal ingot using the Czochralski method. The crucible assembly includes a shell and a liner. The shell is formed of silicon dioxide and has an inner surface and an outer surface opposite to the inner surface. The liner is formed of synthetic silicon dioxide and is formed on the inner surface of the shell. The liner is a thermal sprayed liner and the shell is a cast shell.

There are various improvements of the features mentioned in relation to the above-mentioned aspects. Further features may also be incorporated into the above-mentioned aspects. Such improvements and additional features may exist alone or in any combination. For example, various features discussed below with respect to any of the illustrated embodiments may be incorporated into any of the above-described aspects alone or in any combination.

This application claims the benefit of U.S. Provisional Patent Application No. 62/611,758, filed on December 29, 2017, which is incorporated herein by reference in its entirety.

Referring now to FIG. 1 , a crucible assembly 100 of an example embodiment includes a shell 110 and a liner 120 disposed within the shell 110 such that only the liner 120 contacts the melt. The shell 110 is made of relatively low-grade natural silica using a casting method, and the liner 120 is made of relatively high-grade natural or synthetic silica using a thermal spraying method. The high-grade natural or synthetic silica contributes less impurities to the melt than the low-grade natural silica, and is more expensive than the low-grade natural silica. This provides for reduced overall cost of the crucible assembly 100, while maintaining and/or improving melt quality, compared to crucible assemblies formed entirely from higher grade natural or synthetic silica.

In the example crucible assembly 100, the shell 110 has an inner surface 122 and an outer surface 124 and the liner 120 also has an inner surface 126 and an outer surface 128. The liner 120 is formed on the inner surface 122 of the shell 110 so that the outer surface 128 of the liner 120 conforms to the inner surface 122 of the shell 110. The liner 120 is made of ultra-high purity natural sand or synthetic quartz, and the remainder of the crucible assembly 100 including the shell 110 and the outer surface 124 is made of a lower purity material. The melt material melts within the growth zone 130 defined by the liner 120 and contacts only the inner surface 126 of the liner 120. Thus, the liner 120 prevents the melt from contacting the lower purity material in the housing 110 and improves the quality of the melt.

The shell 110 and liner 120 are bowl-shaped and the liner 120 is heat-sprayed inside the shell 110. The crucible assembly 100 has a diameter 132 extending between the inner surface 126 of the liner 120. In some embodiments, the diameter 132 is at least twenty inches, less than forty inches, between twenty-four inches and thirty-two inches, between twenty-eight inches and thirty-four inches, or between thirty inches and thirty-six inches.

The crucible assembly 100 illustrated in FIG. 1 is formed by a first forming shell 110 by a casting method or a non-casting method. Then, a liner 120 is formed on an inner surface 122 of the shell 110 by a thermal spraying method. Casting methods include a grouting molding method and a glue injection molding method and non-casting methods include an arc fusion method. In an embodiment of the present invention, the shell 110 is formed by a grouting molding method. However, in an alternative embodiment, the shell 110 may be formed by a glue injection molding method or an arc fusion method.

In the illustrated embodiment, the shell 110 is suitably a cast shell, but may be another type of cast shell or crucible. Casting methods suitable for forming cast crucibles typically include pouring a liquid or semi-liquid compound into a mold and allowing the compound to solidify by removing water from the compound. The compound used to form the cast shell 110 may include, for example and without limitation, an aqueous slurry of ceramic powders, such as silica powder. Suitable casting methods for forming cast crucibles include, for example and without limitation, cast molding and glue injection molding. Cast molding includes using an aqueous slurry of a ceramic powder (e.g., silica) known as a slurry. The ceramic powder may be mixed with a dispersant, a binder, water and/or other components. The paste and/or paste mixture (e.g., slurry) is poured into a mold. For example, the mold is suitably made of plaster of Paris (e.g., CaSO ₄ :2H ₂ O). Water from the slurry begins to be removed by capillary action (or with the aid of vacuum drying), and the block is built along the mold wall. When the desired thickness of the dried block is reached, the remainder of the slurry is poured out of the mold. The ceramic green body is then removed from the mold, dried and fired. The firing method includes sintering or fusing at high temperature in the case of silicon dioxide. The final product is opaque at room temperature, but can become transparent depending on the sintering conditions and temperature.

In an alternative embodiment, the shell 110 is made using an injection molding method or other casting method. In the injection molding method, ceramic powder (e.g., natural sand, synthetic quartz or _SiO2 ) is ground and/or mixed with water, a dispersant and a gelling organic monomer. The mixture is placed under a partial vacuum to remove air from the mixture. This increases the drying rate and/or reduces the formation of bubbles in the injection molded product. A catalyst (e.g., a polymerization initiator) is added to the mixture. The polymerization initiator starts a gelling chemical reaction in the mixture. The slurry mixture is cast by pouring the mixture into a mold of the desired shape for producing the product (e.g., a crucible). The mold can be made of, for example, metal, glass, plastic, wax or other materials. The gel is produced from the slurry mixture by heating the mold and slurry mixture in a curing oven. The heat and catalyst cause the monomers in the mixture to form cross-linked polymers, which trap water in the mixture to enlarge the polymer-water gel. The gel bonds and fixes the ceramic particles in the gel. The ceramic is removed from the mold. The ceramic is dried. The dried ceramic can be machined to further shape the ceramic. The ceramic is fired to burn out the polymer in the ceramic and to sinter the ceramic particles. In other alternative embodiments, other casting, machining or production methods are used to make the shell 110.

In another alternative embodiment, the shell 110 is formed by a non-casting method. For example, the shell 110 is formed using an arc fusion method. The method generally includes fusing a precursor material (e.g., high purity silica sand) with an arc. In one embodiment, the shell 110 is formed by pouring high purity silica sand into a rotating mold and then fusing it from the inside out using an arc generated by two or more graphite electrodes. High purity silica sand is defined as sand containing no more than 30 parts per million by weight (ppmw) of impurities. The industry standard for high purity quartz is defined by the product mined and sold as IOTA by Unimin Corporation of Spruce Pine, North Carolina, US, which serves as the high purity benchmark for the high purity quartz market. In this embodiment, the high purity quartz sand has a total impurity content of no more than 20 ppmw. The mold may include vacuum holes through which air trapped between the sand particles and gaseous species generated during the fusion process are removed in order to avoid bubble formation in the final arc-fused crucible. At room temperature, the resulting arc-fused crucible is substantially transparent or translucent, depending on the bubble density.

In an embodiment of the present invention, the shell 110 is formed using a grouting process or another type of casting process to reduce the cost of the crucible, as compared to an arc-fused crucible. The grouting process or another type of casting process provides lower cost design variations (and increased design flexibility) for the shell 110 as compared to the arc-fused process, because the crucible manufactured using the grouting process is made of cheaper, lower grade natural silicon dioxide, rather than more expensive, higher grade natural or synthetic silicon dioxide. In addition, the grouting manufacturing process is less expensive than the arc-fused manufacturing process, as the operating temperature of the arc-fused manufacturing process is significantly higher than the grouting manufacturing process and special equipment is required to achieve the higher operating temperature. Therefore, using a crucible that is cast by injection molding or other methods as the shell 110 in the crucible assembly 100 reduces the cost of the crucible assembly 100 .

The density of the shell 110 formed using the slip casting method or other casting methods can be greater than 90% to 95% of the maximum theoretical density of the silicon dioxide slip casting crucible. The shell 110 formed by the slip casting method and made of silicon dioxide has similar thermal shock resistance characteristics as crucibles formed by other methods (such as amorphous arc fused crucibles). Compared with other types of crucibles (such as hot arc fused crucibles or thermal sprayed crucibles) that are generally transparent or translucent, the cast crucible of this embodiment is opaque at room temperature. It should be noted that the cast crucible of other embodiments may be transparent, for example, depending on the sintering conditions used to fire the cast crucible. Compared to other types of crucibles, such as arc-fused crucibles, cast crucibles generally require additional input power and time to melt the material contained therein due to reduced infrared transmission through the opaque cast crucible compared to transparent or translucent arc-fused crucibles. However, the reduced infrared transmission of the cast crucible can result in less radiant heat loss of the melt after melting compared to arc-fused crucibles. As a result, the total power consumption of a cast crucible may not change throughout the operation compared to an arc-fused crucible. The dissolution rate of a cast crucible is lower than that of an arc-fused crucible.

Additionally, slip cast crucibles typically have a higher impurity content than thermal spray liners. The high impurity content of cast crucibles can result from the ball mill grinding media used to powderize the molten silica feedstock, the mold material used to produce the cast crucible, and impurities and binders and dispersants. Slip cast crucibles include walls of substantially uniform material formed from lower grade natural silica, which include a higher impurity content. This is in contrast to thermal spray liners made from ultra-high purity natural sand or synthetic quartz, which typically have substantially lower impurity contents than slip cast crucibles.

Generally, the shell 110 includes a higher impurity level than a thermal sprayed liner. This is a result of the slurry casting process or other casting process used to make the crucible. In an alternative embodiment, the shell 110 formed by the slurry casting process or other casting process has a low impurity level. For example, the shell 110 has 20 parts per million by weight or less of impurities. Impurities such as aluminum have a significant impact on the low implanted minority carrier lifetime in the crystal and reduce the efficiency of the solar cell made from the crystal. The high purity cast shell 110 reduces impurities and produces a more efficient solar cell.

Cast silica crucibles (such as shell 110) formed from lower grade natural silica are generally not used in Czakowski-based processes because lower grade natural silica has a high total impurity content. Low grade natural silica is typically mined and has 99% by weight silica and 1% by weight impurities. Due to the higher total impurity content, cast crucibles or polysilica cast crucibles are typically used to make polycrystalline silica photovoltaic cells. The higher total impurity content comes from naturally occurring impurities and from impurities added to the silica during the casting process to bond the silica into the cast crucible form. The higher total impurity content of natural silica increases the impurity contribution from the crucible to the melt and to the final product. Cast crucibles or polysilica cast crucibles are typically opaque and have a square bottom. In contrast, higher grade natural silica is typically a refined lower grade natural silica with a sufficiently low impurity content so that it can be used to form a crucible for use in a Czochralski-based process. Crucibles made from higher grade natural or synthetic silica and used in a Czochralski-based process are typically translucent or transparent and have a bowl-shaped bottom.

In some embodiments, the cast shell 110 has an impurity content greater than 50 ppmw, greater than 100 ppmw, greater than 200 ppmw, between 50 ppmw and 1,000 ppmw, between 50 ppmw and 500 ppmw, between 100 ppmw and 1,000 ppmw, between 100 ppmw and 500 ppmw, between 100 ppmw and 400 ppmw, between 200 ppmw and 300 ppmw, greater than 1000 ppmw, or other impurity content greater than the impurity content of thermal spray linings made from ultra-high purity natural sand or synthetic quartz (e.g., having an impurity content less than 0.13 ppmw). Examples of impurities measured or considered in the total impurity content of the shell 110 include, for example, Al, B, Ba, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, P, Ti, Zn, and Zr. For example, the shell 110 may have a total impurity content of less than 230 ppmw and have the following specific impurity contents: 100 ppmw Al, 1 ppmw B, 10 ppmw Ba, 20 ppmw Ca, less than 1 ppmw Cu, 20 ppmw Fe, 15 ppmw K, 10 ppmw Li, 9 ppmw Mg, 23 ppmw Mn, 10 ppmw Na, 10 ppmw Ti, and less than 1 ppmw Zr.

In contrast, a liner formed from the thermally sprayed higher grade natural or synthetic silica of the present invention may have an impurity content that is less than the impurity content of the slip cast shell 110, such as less than 50 ppmw, less than 30 ppmw, less than 20 ppmw, less than 15 ppmw, less than 10 ppmw, less than 1 ppmw, less than 0.5 ppmw, between 0.01 ppmw and 50 ppmw, between 0.01 ppmw and 30 ppmw, between 0.01 ppmw and 20 ppmw, between 5 ppmw and 50 ppmw, between 10 ppmw and 30 ppmw, or other impurity content that is less than the impurity content of the slip cast crucible. For example, a higher grade natural or synthetic silica lining may have a total impurity content of less than 5 ppmw, less than 4 ppmw, less than 3 ppmw, less than 2 ppmw, less than 1 ppmw, or less than 0.13 ppmw with the following specific impurity levels: 0.01 ppmw Al, less than 0.01 ppmw B, 0.01 ppmw Ca, 0.01 ppmw Cr, 0.01 ppmw Cu, 0.02 ppmw Fe, 0.01 ppmw K, 0.01 ppmw Li, 0.01 ppmw Mg, 0.01 ppmw Mn, 0.01 ppmw Na, 0.01 ppmw Ni, less than 0.01 ppmw P, 0.01 ppmw Ti, and 0.01 ppmw Zn. In other embodiments, the thermally sprayed liner has various total impurity levels, various specific impurity levels, and/or other types of impurities.

A crystalline ingot produced by the Czakowski process is pulled from a growth zone 130. The growth zone 130 extends within the inner surface 126 of the liner 120. In operation, the melt contained within the liner 120 and the shell 110 gradually dissolves the inner surface 126 of the liner 120. This dissolution reaction introduces material from the inner surface 126 of the liner 120 into the melt and introduces impurities into the melt. However, because the liner 120 is formed of a higher grade natural or synthetic silicon dioxide, substantially no impurities are introduced from the inner surface 126 of the liner 120. The crucible assembly 100 produces a higher purity crystalline ingot by preventing at least some impurities from entering the growth zone 130 and by preventing those impurities from being incorporated into the crystalline ingot. The crucible assembly 100 benefits from the increased design flexibility, reduced cost, and increased crucible life provided by the cast shell 110 while reducing the effects of impurities in the cast shell 110.

2, a flow chart illustrates an example method 200 for using a slip casting process to make a crucible for use in the crucible assembly 100 shown in FIG1. This and/or other methods are used to make the shell 110. The method 200 generally includes mixing 202 silicon dioxide with other components to form a slurry, pouring 204 the slurry into a mold, drying 206 the slurry and/or the mold to form a green body, removing 208 the green body from the mold, firing 210 the green body, and cooling 212 the green body.

The step of mixing 202 the silica and other components to form a slurry includes mixing the silica with a dispersant, a binder and/or water to form a slurry. The mixed silica can be wet-ground molten silica. Casting 204 the slurry into a mold includes pouring the slurry mixture into the mold. The mold is typically made of plaster of Paris. In an embodiment using injection molding rather than injection molding, the mold is, for example, stainless steel. The step of drying 206 the slurry and/or mold to form a green body includes removing water from the slurry by capillary action with or without the assistance of vacuum drying. The green body is in the form of a powder that has not been fired and formed. During the drying of the slurry, dry lumps are formed along the walls of the mold. When the desired thickness of the dried cake is reached, the remaining liquid slurry is poured out. Firing 210 the green body includes sintering or fusing the dried cake, for example, silicon dioxide within the dried cake, at a high temperature.

The liner 120 is a thermally sprayed liner formed by a thermal spraying method. The method generally includes spraying a liquid or semi-liquid compound onto the shell 110 and allowing the compound to solidify on the inner surface 122 of the shell 110. In one embodiment, the liner 120 is formed by melting higher-grade natural or synthetic silica or high-purity quartz sand and spraying the molten higher-grade natural or synthetic silica on the inner surface 122 of the shell 110. High-purity quartz sand is defined as sand particles containing no more than 30 ppmw of impurities. The industry standard for high purity quartz is defined by the product mined and sold as IOTA by Unimin Corporation of Spruce Pine, North Carolina, US, which serves as the high purity benchmark for the high purity quartz market. In this embodiment, the higher grade natural or synthetic silica or synthetic quartz has a total impurity content of no more than 0.13 ppmw. In some embodiments, the higher grade natural or synthetic silica or synthetic quartz has a total impurity content of less than 5 ppmw, less than 4 ppmw, less than 3 ppmw, less than 2 ppmw, less than 1 ppmw, or less than or equal to 0.13 ppmw.

In a batch or recharge Czakowski process, ultra-high purity natural sand or synthetic quartz (e.g., SiO ₂ ) can be used for lining 120 that is in contact with molten silicon in growth zone 130, while shell 110 is made of lower purity sand. This configuration can also be used for a continuous Czakowski process. Ultra-high purity natural sand has a higher purity than high purity natural sand, such as not more than 0.13 ppmw. Synthetic quartz has a higher purity than ultra-high purity natural sand, such as not more than 0.13 ppmw.

In an alternative embodiment, both the liner 120 and the shell 110 can be formed from ultra-high purity natural sand or synthetic quartz. In yet another alternative embodiment, the entire crucible assembly 100 is made of a single material or is made primarily of a single material. For example, the crucible assembly 100 can be made entirely of ultra-high purity natural sand or synthetic quartz with less than 20 parts per million by weight of impurities.

As described herein, thermal spraying methods generally describe many methods that are broadly classified into three thermal spraying method categories: flame thermal spraying methods, electric thermal spraying methods, and dynamic thermal spraying methods. Each category of thermal spraying methods melts or propels the coating compound in a unique manner. For example, flame thermal spraying methods typically melt the coating compound with a flame, while electric thermal spraying methods use an electric current to melt the coating compound. Dynamic thermal spraying methods typically propel the coating compound at extremely high speeds, causing the compound to deform and bond upon impact. All thermal spraying methods generally require a spray torch, a coating compound, and energy to melt or propel the coating compound.

Referring now to FIG. 3 , a block diagram of a thermal spray assembly 300 applicable to all thermal spray methods is illustrated. The thermal spray assembly 300 generally includes a torch or spray gun 302, an energy source 304, a coating compound source 306, an accelerating medium source 308, and optionally, a cooling medium source 310. The coating compound source 306 provides the coating compound to the spray gun 302. In this embodiment, the coating compound is a higher grade natural or synthetic silicon dioxide. In an alternative embodiment, the coating compound is ultra-high purity natural sand, high purity natural sand, or any coating compound that enables the crucible assembly 100 to operate as described herein. The energy source 304 provides energy to melt the coating compound into molten particles, which are then sprayed onto the housing 110. The accelerating medium source 308 provides a suitable medium for accelerating the molten particles of the coating compound toward the housing 110. In some embodiments, the energy source 304 and the accelerating medium source 308 are combined into a single energy and accelerating medium source. In some embodiments, the cooling medium source 310 provides a cooling medium (typically water) to cool the spray gun 302 during operation.

4, a flow chart illustrates a method 400 for thermally spraying a liner 120 onto a shell 110. The method 400 generally includes providing 402 a thermal spray assembly 300, pre-treating 404 an inner surface 122 of the shell 110, providing 406 a coating compound from a coating compound source 306, providing 408 energy from an energy source 304, using the energy from the energy source 304 to melt 410 the coating compound to form molten particles of the coating compound, providing 412 an accelerating medium from an accelerating medium source 308, using the accelerating medium from the accelerating medium source 308 to move the molten particles of the coating compound toward the shell 110, and applying 414 a heat treatment process to the coating compound. The method 400 includes accelerating 414 the shell 110, spraying 416 molten particles of the coating compound and the accelerating medium toward the inner surface 122 of the shell 110 using the torch 302 to form a coating layer 312 of the coating compound on the inner surface 122, moving 418 the torch 302 in the direction 314 to form more coating layers 312, bonding 420 the coating 312 to the inner surface 122 to form the liner 120, and post-treating 422 the inner surface 126 of the liner 120 and/or the shell 110 with or without plasma spraying. The method 400 may also optionally include providing 424 a cooling medium from the cooling medium source 310. Providing 402 the thermal spray assembly 200 generally includes providing a spray torch 302, an energy source 304, a coating compound source 306, an acceleration medium source 308, and a cooling medium source 310. Providing 404 the coating compound from the coating compound source 306 includes providing a higher level natural or synthetic silicon dioxide.

Pre-treating 404 the inner surface 122 of the shell 110 is a pre-treatment method that can improve the bonding, deposition and/or coating of the liner 120 to the shell 110 and/or improve the quality of the liner 120. Pre-treating 404 can include any method of pre-heating the inner surface 122 of the shell 110, chemically pre-treating the inner surface 122 of the shell 110, roughness pre-treatment on the inner surface 122 of the shell 110, and/or improving the adhesion of the liner 120 to the shell 110. Any of the pre-treatment methods listed above can be used alone or in combination with any other pre-treatment method.

The preheating pretreatment method generally includes preheating the inner surface 122 of the shell 110 before the liner 120 is thermally sprayed onto the shell 110. A preheating device (such as a boiler or a torch) can be used to preheat the inner surface 122 of the shell 110. In addition, when the flame thermal spraying method is used to thermally spray the liner 120, the thermal spraying assembly 300 can be used to preheat the inner surface 122 of the shell 110. Specifically, the thermal spraying assembly 300 can pre-spray the inner surface 122 of the shell 110 without any coating compound, so that the flame that is usually used to melt the coating compound is used instead to preheat the inner surface 122 of the shell 110. The preheating method may include any method or device that preheats the inner surface 122 of the housing 110 to improve the adhesion of the liner 120 to the housing 110.

The chemical pretreatment method generally includes chemically preheating the inner surface 122 of the shell 110 before thermally spraying the liner 120 onto the shell 110. A chemical pretreatment device (such as a spraying device or other chemical coating device) can be used to pretreat the inner surface 122 of the shell 110. In addition, a thermal spraying assembly 300 can be used to chemically pretreat the inner surface 122 of the shell 110. Specifically, the thermal spraying assembly 300 can be used to spray a pretreatment coating that improves the adhesion of the liner 120 to the shell 110. The pretreatment coating may include a paste or a solvent that improves the adhesion of the liner 120 to the shell 110. The chemical pre-treatment method may include any method or device that pre-treats the housing 110 to improve the adhesion of the liner 120 to the housing 110.

The roughness pretreatment method generally includes mechanically changing the roughness of the inner surface 122 of the shell 110 before the liner 120 is heat sprayed onto the shell 110. A roughness pretreatment device (such as a sanding device or other mechanical surface treatment device) can be used to pre-treat the inner surface 122 of the shell 110. The mechanical surface treatment device can be used to increase or decrease the roughness of the inner surface 122 of the shell 110 to improve the adhesion of the liner 120 to the shell 110. The roughness pretreatment method can include any method or device for pre-treating the shell 110 to improve the adhesion of the liner 120 to the shell 110.

After the liner 120 has been heat sprayed onto the shell 110, the liner 120 is cooled and cured to form a glassy silicon dioxide layer. To ensure that a glassy, amorphous silicon oxide layer is formed, the cooling and curing methods are controlled or optimized. Specifically, post-treatment 422 of the inner surface 126 of the liner 120 and/or the shell 110 is a post-treatment method that is optionally selected to control or optimize the cooling and/or curing methods. Post-treatment 422 may include a cooling rate post-treatment method, a chemical post-treatment method, a plasma spray post-treatment method, and/or any method that improves the adhesion of the liner 120 to the shell 110. Any of the post-treatment methods listed above may be used alone or in combination with any other pre-treatment method.

The cooling post-processing method generally includes controlling or optimizing the cooling rate of the liner 120 and/or the shell 110 to improve the adhesion of the liner 120 to the shell 110. The post-processing cooling method may include using a cooling device (such as a fan or blower) to cool the liner 120 and/or the shell 110 to increase the cooling rate of the liner 120 and/or the shell 110 to improve the adhesion of the liner 120 to the shell 110. The post-processing cooling method may also include cooling the liner 120 and/or the shell 110 by introducing a low temperature gas toward the inner surface 126 of the liner 120 to increase the cooling rate of the liner 120 and/or the shell 110 to improve the adhesion of the liner 120 to the shell 110. The post-processing cooling method may also include heating the liner 120 and/or the shell 110 using a heating device (such as a boiler or a torch) to reduce the cooling rate of the liner 120 and/or the shell 110 to improve the adhesion of the liner 120 to the shell 110. The post-processing cooling method may include any method or device for controlling, optimizing, increasing and/or decreasing the cooling rate of the liner 120 and/or the shell 110 to improve the bonding, deposition and/or spraying of the liner 120 and the shell 110 and/or the quality of the liner 120.

The chemical post-treatment method generally includes chemically post-treating the liner 120 and/or the shell 110 after the liner 120 is thermally sprayed onto the shell 110. A chemical post-treatment device (such as a spraying device or other chemical coating device) can be used to post-treat the liner 120 and/or the shell 110. In addition, a thermal spray assembly 300 can be used to chemically post-treat the liner 120 and/or the shell 110. Specifically, the thermal spray assembly 300 can be used to spray a post-treatment coating that improves the adhesion of the liner 120 to the shell 110 onto the liner 120 and/or the shell 110. The post-treatment coating may include a paste or solvent that improves the adhesion of the liner 120 to the shell 110. The chemical post-treatment method may include any method or device that post-treats the shell 110 to improve the adhesion of the liner 120 to the shell 110.

The thermal post-treatment method generally includes heat treating the inner surface 126 of the liner 120 after the liner 120 is heat sprayed onto the housing 110. A heat treatment device such as a plasma torch may be used to heat treat the inner surface 126 of the liner 120. In addition, when the liner 120 is heat sprayed using a plasma heat spraying method, the heat spraying assembly 300 may be used to heat treat the inner surface 126 of the liner 120. Specifically, the heat spraying assembly 300 may spray the inner surface 126 of the liner 120 without any coating compound, so that plasma spraying, which is usually used to melt the coating compound, is used instead to heat treat the inner surface 126 of the liner 120. The thermal post-treatment method may include any method or device that thermally treats the inner surface 122 of the housing 110 to improve the adhesion of the liner 120 to the housing 110.

In addition, any of the above-listed post-processing methods may be used in combination with any other post-processing method to ensure that the liner 120 is formed as amorphous glass rather than crystallized glass. Specifically, if the temperature of the heat-sprayed liner 120 is too high and its cooling rate is too low, the liner 120 will form a crystallized glass liner 120 rather than an amorphous glass liner 120. In this way, a combination of post-processing methods may be used to control the cooling rate of the liner 120. For example, at the end of the heat-spraying method, a thermal post-processing method may be used to maintain the high temperature of the liner 120 until the cooling post-processing method is started to control the cooling rate of the liner 120. Specifically, the thermal spray assembly 300 may spray the inner surface 126 of the liner 120 with plasma spraying to maintain the temperature of the liner 120 immediately after the liner 120 has been thermally sprayed onto the housing 110. The thermal spray assembly 300 maintains the temperature of the liner 120 until a post-cooling treatment process is initiated to control the cooling rate of the liner 120. The post-cooling treatment process may include a cooling surface positioned below the housing 110 or introducing a low temperature gas toward the inner surface 126 of the liner 120 to increase the cooling rate of the liner 120.

One or more intermediate steps may be inserted between any of the steps of method 400 to improve the adhesion of liner 120 to housing 110. The intermediate steps may include any method or device that improves the adhesion of liner 120 to housing 110.

Higher grade natural or synthetic silicon dioxide has a high melting point (about 1710° C.). Thus, a large amount of energy is required to melt synthetic silicon dioxide and it is difficult to use a small thermal spraying device such as thermal spraying assembly 300 to soften the silicon dioxide enough to form a coating. Therefore, energy source 304 must provide sufficient energy to soften and melt the synthetic silicon dioxide into molten particles.

Additionally, contaminants may be introduced into the molten particles of the coating compound during the thermal spraying process by the thermal spraying assembly 300. The thermal spraying assembly 300 may be designed or configured to reduce or eliminate contaminants introduced into the molten particles of the coating compound by the thermal spraying assembly 300 during the thermal spraying process.

In flame thermal spraying methods, providing 406 energy from energy source 304 generally includes providing a gaseous chemical substance capable of providing energy from an oxidation reaction. That is, flame thermal spraying methods generally provide energy and melt the coating compound by burning hydrocarbons (such as but not limited to acetylene, kerosene, or natural gas) in the presence of oxygen or compressed air. Flame thermal spraying methods generally include detonation spraying methods, line-fire spraying methods, flame powder spraying methods, high velocity oxygen fuel (HVOF) spraying methods, and high velocity air fuel (HVAF) spraying methods.

In the detonation spraying method, powered synthetic silicon dioxide is provided from a coating compound source 306 and injected into the long barrel of the torch 302. The long barrel is water-cooled with water provided from a cooling medium source 310. Oxygen and a hydrocarbon fuel such as acetylene are also injected into the barrel and detonated using an ignition mechanism. The detonation melts the powdered synthetic silicon dioxide and accelerates the molten synthetic silicon dioxide along with the resulting combustion gases out of the long barrel and onto the inner surface 122. The detonation is repeated multiple times per second.

In the flame spraying method, synthetic silicon dioxide in the form of a wire is fed into the torch 302 while oxygen and a hydrocarbon fuel such as acetylene are burned to melt the synthetic silicon dioxide. Compressed air is also provided to atomize the molten particles of the synthetic silicon dioxide and accelerate the molten silicon dioxide toward the inner surface 122.

In the flame powder spraying method, synthetic silicon dioxide in powder form is fed into the torch 302 while oxygen and a hydrocarbon fuel such as acetylene are burned to melt the synthetic silicon dioxide. Compressed air is mixed with the powdered synthetic silicon dioxide to transport the silicon dioxide into the flame. The resulting combustion gas and molten particles of synthetic silicon dioxide are accelerated toward the inner surface 122 by the compressed air.

In the HVOF spraying method, the torch 302 includes a combustion chamber and a nozzle. Oxygen and a hydrocarbon fuel such as propylene are fed into the combustion chamber and ignited. The resulting combustion gas is fed through the nozzle to form an ultrasonic flame, which is then fed into the barrel of the torch 302 at a high speed. In some embodiments, the exit velocity of the ultrasonic flame from the barrel exceeds the speed of sound. Synthetic silicon dioxide in powder form is entrained in a carrier gas (usually nitrogen) and injected into the barrel of the torch 302 together with the ultrasonic flame. The ultrasonic flame melts the synthetic silicon dioxide into molten particles of synthetic silicon dioxide and accelerates the molten particles of synthetic silicon dioxide toward the inner surface 122. Cooling water is typically provided to cool the torch 302.

The HVAF spraying process is similar to the HVOF spraying process, except that compressed air rather than oxygen is fed into the combustion chamber and ignited, producing a low temperature ultrasonic flame.

In the electrothermal spraying method, providing energy 406 from the energy source 304 generally includes providing an electric current for directly or indirectly melting and forming silicon dioxide. The electrothermal spraying method generally includes a plasma spraying method and an arc wire spraying method.

In the plasma jetting method, the torch 302 includes an electrode and a nozzle positioned adjacent thereto so that a high frequency or high voltage arc is formed therebetween. An inert gas (usually argon) flows between the electrode and the nozzle and is ionized by the arc. The ionization of the inert gas produces a plasma with an elevated temperature and velocity. Synthetic silicon dioxide in powder form is entrained in the plasma in which it is melted into molten particles of synthetic silicon dioxide and accelerated toward the inner surface 122.

In the arc wire spraying process, two wires of synthetic silicon dioxide are fed into the spray torch 302 and current is fed to each wire. The wires are brought into close proximity with each other so that the current in the short circuit between the two wires raises the temperature of the wires. The raised temperature melts the tops of the wires and compressed air or inert gas passes through the molten tops of the wires to atomize molten particles of synthetic silicon dioxide and accelerate them toward the inner surface 122.

In a kinetic thermal spraying method, providing energy from an energy source 304 generally includes providing a high velocity atomized gas stream to accelerate the coating compound to an extremely high velocity. The kinetic thermal spraying method generally includes a variation of a cold gas spraying method. In a cold gas spraying method, synthetic silicon dioxide in powder form is entrained in a high velocity atomized gas stream. The atomized gas is heated and partially melts the synthetic silicon dioxide. Once entrained, the high velocity atomized gas accelerates the powdered synthetic silicon dioxide toward the inner surface 122 at a velocity of more than 1,000 meters per second. The extremely high velocity causes the powdered partially melted synthetic silicon dioxide to deform and mechanically bond to the inner surface 122 after impact with the inner surface 122, thereby producing the lining 120.

Referring now to FIG. 5 , a flow chart illustrates a method 500 for making the crucible assembly shown in FIG. 1 . The method 500 generally includes forming 502 a shell 110 using a slip injection molding method and forming 504 a liner 120 using a thermal spraying method. Forming 502 the shell 110 using the slip injection molding method includes forming the shell 110 according to the method 200 illustrated in FIG. 2 . Forming 504 the liner 120 using the thermal spraying method includes forming the liner 120 according to the method 400 illustrated in FIG. 4 . In alternative embodiments, the shell 110 is formed using alternative methods such as slip injection molding.

6, a flow chart illustrates a method 600 for pulling a crystalline ingot using the Czakowski method and the crucible assembly 100 shown in FIG1. The method 600 generally includes providing 602 a crucible assembly 100 including a liner 120 and a shell 110, melting 604 a semiconductor material and/or a solar grade material in the crucible assembly 100, pulling 606 a single crystal of the semiconductor and/or solar grade material from the crucible assembly 100, and feeding 608 the semiconductor and/or solar grade material into the crucible assembly 100.

As shown in FIG. 1 , a crucible assembly 100 provided for use in method 600 includes a liner 120 formed within a housing 110. Melting 604 semiconductor material and/or solar grade material in crucible assembly 100 includes melting the material in growth zone 130. After melting 604 the material, the molten material at least partially fills growth zone 130. Pulling 606 a single crystal of semiconductor and/or solar grade material from crucible assembly 100 includes pulling 606 a single crystal from growth zone 130 within liner 120. Feeding 608 semiconductor and/or solar grade material into crucible assembly 100 includes adding additional material to growth zone 130.

7, a cross-sectional view of a crucible assembly 700 illustrates an alternative embodiment of the crucible assembly 100. The crucible assembly 700 includes a liner 720 thermally sprayed on a portion of the shell 710. In contrast to the crucible assembly 100, the liner 720 is thermally sprayed only on the wetted surface of the crucible assembly 700. Reducing the area of the liner 720 reduces the cost of the crucible assembly 700 compared to the crucible assembly 100.

The resulting crucible assemblies of the present invention result in reduced cost, improved design flexibility, improved crucible life, and limited impurities introduced into single crystal ingots extracted from the crucible assembly. In some embodiments, the crucible assembly reduces cost by using a cast molded shell. The reduced cost of cast molding and its use for larger shells results in reduced cost compared to arc fusion. The cost of producing a cast crucible is less than that of producing an arc fused crucible because the capital equipment used to produce a cast crucible is less expensive than the capital equipment used to produce an arc fused crucible, and the energy required to produce a cast crucible is less than the energy required for an arc fused crucible. Additionally, a shell made from high impurity, less expensive natural silica rather than lower impurity, higher cost synthetic silica requires the manufacture of high quality ingots. Using less expensive materials to form the bulk of the crucible substantially reduces costs.

The crucible assemblies of some embodiments have improved design flexibility due to the use of a cast shell. Compared to the equipment used to produce arc-fused crucibles, such as rotating molds, electrodes, etc., the molds used to produce cast crucibles can be easily and cheaply changed to produce different crucible geometries, such as larger or smaller diameter crucibles. Finally, the liner disclosed herein acts as a low-impurity barrier between the melt and the high-impurity shell. This limitation is introduced into the impurities in the single crystal ingot extracted from the crucible assembly using the liner.

When introducing elements or embodiments of the present invention, the articles "a," "an," and "the," are intended to mean that there are one or more elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of terms indicating a particular orientation (e.g., "top," "bottom," "side," "lower," "upper," etc.) is for convenience of description and does not require any particular orientation of the items described.

As various changes could be made in the above constructions and methods without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

100‧‧‧Crucible assembly 110‧‧‧Shell 120‧‧‧Lining 122‧‧‧Inner surface of shell 124‧‧‧Outer surface of shell 126‧‧‧Inner surface of lining 128‧‧‧Outer surface of lining 130‧‧‧Growth zone 132‧‧‧Diameter of crucible assembly 200‧‧‧Method 202‧‧‧Mixed 204‧‧‧Casting 206‧‧‧Drying 208‧‧‧Removal 210‧‧‧Firing 212‧‧‧Cooling 300‧‧‧Hot spray assembly 302‧‧‧Torch or spray gun 304‧‧‧Energy source 306‧‧‧Coating compound source 308‧‧‧Accelerating medium source 310‧‧‧Cooling medium source 312‧ ‧‧Coating of coating compound 314‧‧‧Direction of moving torch 400‧‧‧Method 402‧‧‧Provide 404‧‧‧Pretreatment 406‧‧‧Provide 408‧‧‧Provide 410‧‧‧Melting 412‧‧‧Provide 414‧‧‧Acceleration 416‧‧‧Spraying 418‧‧‧Move 420‧‧‧ Bonding 422‧‧‧Post-treatment 424‧‧‧Provide 500‧‧‧Method 502‧‧‧Form 504‧‧‧Form 600‧‧‧Method 602‧‧‧Provide 604‧‧‧Melting 606‧‧‧Drawing 608‧‧‧Feeding 700‧‧‧Crucible assembly 710‧‧‧Shell 720‧‧‧Lining

FIG. 1 is a cross-sectional view of a crucible assembly including a body and a liner.

FIG. 2 is a flow chart illustrating a suitable method for making the housing shown in FIG. 1 .

Figure 3 is a block diagram of the thermal spray assembly.

4A-4B are flow charts illustrating a suitable method for using the thermal spray assembly shown in FIG. 3 .

FIG. 5 is a flow chart illustrating a suitable method for making the crucible assembly shown in FIG. 1 .

FIG. 6 is a flow chart illustrating a suitable method for pulling a crystalline ingot using the Czochralski method and the crucible assembly shown in FIG. 1 .

FIG. 7 is a cross-sectional view of another crucible assembly including a main body and a liner.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

100‧‧‧Crucible components

110‧‧‧Shell

120‧‧‧lining

122‧‧‧Inner surface of the shell

124‧‧‧Outer surface of the shell

126‧‧‧Inner surface of lining

128‧‧‧Outside surface of lining

130‧‧‧Growth Area

132‧‧‧Diameter of crucible assembly

Claims (19)

A method for manufacturing a crucible assembly having a shell and a liner, the method comprising: using a casting method to form the shell, the shell comprising silicon dioxide and having an inner surface and an outer surface; forming the liner on the inner surface of the shell, the liner being formed of synthetic silicon dioxide, wherein the synthetic silicon dioxide has a total impurity content of less than 0.13 ppmw, wherein forming the liner on the inner surface of the shell comprises forming the liner on the inner surface of the shell by a thermal spraying method, the thermal spraying method comprising spraying the synthetic silicon dioxide on the inner surface of the shell at a high temperature. As in the method of claim 1, wherein the casting method is a grouting method. The method of claim 2, wherein the injection molding method comprises: mixing molten silica, water, a dispersant and a binder to form a slurry; casting the slurry into a mold; drying the slurry; removing the dried slurry from the mold to form a green body; and firing the green body. As in the method of claim 3, sintering the green body includes sintering the green body at a high temperature. The method of claim 3, wherein the mold comprises plaster of Paris. The method of claim 3, wherein drying the slurry comprises vacuum drying the slurry. The method of claim 1, wherein the thermal spraying method comprises: melting the synthetic silicon dioxide to form molten particles of the synthetic silicon dioxide; using an accelerating medium to accelerate the molten particles of the synthetic silicon dioxide toward the shell; using a torch to spray the molten particles of the synthetic silicon dioxide and the accelerating medium toward the inner surface of the shell; forming a coating of the synthetic silicon dioxide on the inner surface of the shell; and bonding the coating of the synthetic silicon dioxide to the shell to form the lining. The method of claim 7 further comprises moving the torch in the first direction to form more of the coating layer. The method of claim 7 further comprises providing a cooling medium to cool the torch. The method of claim 7 further comprises providing the accelerating medium from an accelerating medium source. The method of claim 7 further comprises providing the synthetic silica in the form of powdered synthetic silica. The method of claim 7 further comprises providing the synthetic silica in the form of synthetic silica filaments. The method of claim 7, wherein melting the synthetic silicon dioxide to form molten particles of the synthetic silicon dioxide comprises melting the synthetic silicon dioxide with a flame. The method of claim 7, wherein melting the synthetic silicon dioxide to form molten particles of the synthetic silicon dioxide comprises melting the synthetic silicon dioxide with an electric current. The method of claim 7, wherein melting the synthetic silicon dioxide to form molten particles of the synthetic silicon dioxide comprises melting the synthetic silicon dioxide using ionized plasma. The method of claim 7, wherein melting the synthetic silicon dioxide to form molten particles of the synthetic silicon dioxide comprises partially melting the synthetic silicon dioxide with a heated accelerating medium. The method of claim 16, wherein using an accelerating medium to accelerate the molten particles of the synthetic silicon dioxide toward the shell includes accelerating the partially molten synthetic silicon dioxide toward the shell at a high velocity using the heated accelerating medium. As in the method of claim 1, wherein the casting method is an injection molding method. A crucible assembly for growing a crystalline ingot using the Czochralski method, the assembly comprising: a shell formed of silicon dioxide and having an inner surface and an outer surface opposite to the inner surface; and a liner formed of synthetic silicon dioxide and formed on the inner surface of the shell, wherein the liner is a thermally sprayed liner and the shell is a cast shell, wherein the synthetic silicon dioxide has a total impurity content of less than 0.13 ppmw.